1. Field of the Invention
The present invention relates to switching power supply circuits, and more particularly, the invention relates to switching power supply circuits in which heat generation and power loss in the switching element can be reduced by performing intermittent switching operations under light load.
2. Description of the Related Art
FIG. 3 shows a conventional switching power supply circuit disclosed in Japanese Unexamined Patent Application Publication No. 4-12666, which can perform intermittent switching operations under light load.
In FIG. 3, a switching power supply circuit 1 has a DC power supply E, a switching unit 2, a switching control unit 3, a transformer T which includes a primary winding N1, a secondary winding N2, and a feedback winding Nb, a rectifying/smoothing circuit 4, a first photo coupler (PC) 5, a current detecting unit 6, an intermittent/continuous switching signal generating unit 7, and a second photo coupler 8. In this case, the power supply E is connected to the primary winding N1 of the transformer T via the switching unit 2. The switching control unit 3 is connected to the feedback winding Nb of the transformer T, and is also connected to the switching unit 2. The secondary winding N2 of the transformer T is connected to the rectifying/smoothing circuit 4, whose outputs are connected to the switching control unit 3 via the first photo coupler 5, and are also connected to output terminals Po via the current detecting unit 6. The current detecting unit 6 is connected to the intermittent/continuous switching signal generating unit 7, whose output is connected to the switching control unit 3 via the second photo coupler 8.
In the switching power supply unit 1 having such a structure, an output current is detected by the current detecting unit 6. First, when the current value detected by the current detecting unit 6 is large, that is, under rated load or heavy load, the intermittent/continuous switching signal generating unit 7 outputs a signal for directing continuous operations. This signal is input to the switching control unit 3 via the second photo coupler 8, and, according to the control of the switching control unit 3, the switching unit 2 performs continuous switching operations.
Meanwhile, when the current value detected by the current detecting unit 6 is small, that is, under light load, the intermittent/continuous switching signal generating unit 7 outputs a signal for directing intermittent operations. This signal is input to the switching control unit 3 via the second photo coupler 8, and, according to the control of the switching control unit 3, the switching unit 2 performs intermittent switching operations.
Usually, there are problems in that since the switching frequency significantly rises under light load, the switching element generates heat and switching loss increases. However, these problems can be solved by performing intermittent switching operations as described above.
In the switching power supply circuit 1, however, the signals for directing continuous operations and intermittent operations are output by using the current detecting unit 6 and the intermittent/continuous switching signal generating unit 7, and the output signals are fed back to the switching control unit 3 to perform the switching of intermittent/continuous operations. In this case, since it is necessary to dispose the current detecting unit 6 and intermittent/continuous switching signal generating unit 7, there are problems in that the structure of the switching power supply unit becomes complicated, and miniaturization of the circuit is thereby hindered and cost increases.